Annual report pursuant to Section 13 and 15(d)

ACQUISITION OF FIRSTMERIT CORPORATION - Narrative (Details)

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ACQUISITION OF FIRSTMERIT CORPORATION - Narrative (Details)
$ / shares in Units, $ in Thousands, shares in Millions
 3 Months Ended 12 Months Ended
Aug. 16, 2016
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
state
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Sep. 30, 2016
USD ($)
Jun. 30, 2016
USD ($)
Mar. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
state
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Aug. 15, 2016
$ / shares
Business Acquisition [Line Items]                          
Goodwill   $ 1,993,000       $ 1,993,000       $ 1,993,000 $ 1,993,000 $ 677,000  
Number of States in which Entity Operates | state   8               8      
Share Price | $ / shares                         $ 9.68
Net Income (Loss) Attributable to Parent   $ 432,000 $ 275,000 $ 272,000 $ 207,000 $ 239,000 $ 127,000 $ 175,000 $ 171,000 $ 1,186,000 $ 712,000 $ 693,000  
FirstMerit Bank                          
Business Acquisition [Line Items]                          
Business Combination, Consideration Transferred $ 3,700,000                        
Payments to acquired business in cash 800,000                        
Goodwill $ 1,000,000                        
Business Combination, Consideration Transferred, Equity Interests Issued and Issuable, Per Share | $ / shares $ 1.72                        
Business Combination, Consideration Transferred, Cash Paid for Each Share of Acquired Entity's Shares | $ / shares $ 5.00                        
Consideration transferred, equity interest issued and issuable $ 2,800,000                        
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares | shares 285                        
Common Stock | FirstMerit Bank                          
Business Acquisition [Line Items]                          
Consideration transferred, equity interest issued and issuable $ 2,800,000                        
Preferred Stock | FirstMerit Bank                          
Business Acquisition [Line Items]                          
Consideration transferred, equity interest issued and issuable $ 100,000